1. Field of the Invention
This invention pertains to subsea pipelines. More particularly, apparatus is provided for electrically insulating and connecting electrical power to a segment of a pipeline that is electrically heated using a pipe-in-pipe configuration.
2. Description of Related Art
Offshore hydrocarbon recovery operations are increasingly moving into deeper water and more remote locations. Often satellite wells are completed at the sea floor and are tied to remote platforms or other facilities through extended subsea pipelines. Some of these pipelines extend through water that is thousands of feet deep and where temperatures of the water near the sea floor are in the range of 40xc2x0 F. The hydrocarbon fluids, usually produced along with some water, reach the sea floor at much higher temperatures, characteristic of depths thousands of feet below the sea floor. When the hydrocarbon fluids and any water present begin to cool, phenomena occur that may significantly affect flow of the fluids through the pipelines. At these low temperatures, some crude oils become very viscous or deposit paraffin. Either phenomenon can prevent flow. Hydrocarbon gas under pressure (free gas or solution gas in crude oil) can combine with water at reduced temperatures to form a solid ice-like material, called a xe2x80x9chydrate.xe2x80x9d Hydrates can plug pipelines and the plugs are very difficult to remove. In deep water, conventional methods of depressuring the flow line to remove a hydrate plug may not be effective. Higher pressures in the line and uneven sea floor topography may cause excessive time requirements for remediation, which can be costly in terms of lost production.
The problem of lower temperatures in subsea pipelines has been addressed by placing thermal insulation on the lines, but the length of some pipelines makes thermal insulation alone ineffective. Increased flow rate through the lines also helps to minimize temperature loss of the fluids, but flow rate varies and is determined by other factors. Problems of heat loss from a pipeline increase late in the life of a hydrocarbon reservoir because production rates often decline at that time. Problems become particularly acute when a pipeline must be shut-in for an extended period of time. This may occur, for example, because of work on the wells or on facilities receiving fluids from the pipeline. The cost of thermal insulation alone to prevent excessive cooling of the lines becomes prohibitive under these conditions.
Heating of pipelines by bundling the lines with a separate pipeline that can be heated by circulation of hot fluids has been long practiced in the industry. Also, heating by a variety of electrical methods has been known. Most of the proposals for electrical heating of pipelines have related to pipelines on land, but in recent years industry has investigated a variety of methods for electrical heating of subsea pipelines. (xe2x80x9cDirect Impedance Heating of Deepwater Flowlines,xe2x80x9d OTC 11037, May, 1999)
Two configurations for electrical heating have been considered. In one configuration, a single flowline is electrically insulated and current flows along the flowline. This is called the xe2x80x9cSHIPxe2x80x9d system (Single Heated Insulated Pipe). In the second configuration for electrical heating, a pipe-in-pipe subsea pipeline is provided by which a flow line for transporting well fluids is the inner pipe and it is surrounded concentrically by and electrically insulated from an electrically conductive outer pipe until the two pipes are electrically connected at one end. Voltage is applied between the inner and outer pipes at the opposite end and electrical current flows along the exterior surface of the inner pipe and along the interior surface of the outer pipe. This pipe-in-pipe method of heating is disclosed, for example, in U.S. Pat. No. 6,142,707, which is commonly assigned and hereby incorporated by reference herein. Other patents related to the pipe-in-pipe method of heating include U.S. Pat. No. 6,292,627 B1 and U.S. Pat. No. 6,371,693 B1, which are hereby incorporated by reference.
Any method of electrical heating of a segment of a pipeline requires that the segment be electrically insulated from other parts of the pipeline. The pipe-in-pipe method of heating disclosed in the referenced patents requires, when power is applied at one end of the segment to be heated, an Electrical Insulating Joint (herein xe2x80x9cEIJxe2x80x9d) at the powered end of the segment. The powered end is normally on or attached to an offshore platform or other structure where electrical power is generated. The voltage drop across the EIJ determines the amount of heating available and the length of a segment that can be heated; for a pipeline a few miles long a voltage drop of thousands of volts is expected. Electrical currents through the pipeline may be in the range of hundreds of amperes.
subsea pipeline may contain, along with hydrocarbons, water, grease, pipe dope, well treating chemicals, inhibitors or other contaminants and, from time-to-time, even metallic parts from subsurface equipment such as sand screens or chokes. Water may condense above the EIJ as fluids in the heated segment cool. Therefore, there is need for an electrical insulating joint that can maintain electrical isolation even in the presence of harsh chemical and mechanical environments. The insulating joint should be able to survive repeated exposure to all these materials without failing electrically or reducing the heating capability of the system. The primary protection should be passive, i.e., not dependent on instrumentation, but instrumentation may be used for monitoring. The device should also be capable of transmitting the large static loads of a subsea pipeline riser that is tied to the structure. Under no circumstances should there be a pressure release or exposure of an ignition source.
Apparatus is provided for applying electrical power to a pipe-in-pipe heated pipeline. An Electrical Insulating Joint (EIJ) provides mechanical joining, pressure containment and electrical isolation of a heated and an unheated portion. A ceramic ring under compression and dielectrics in the annulus separate inner and outer pipe hubs. A dielectric liner is placed over the ceramic ring and the wall of the flow channel for a selected distance in each direction from the ceramic ring. Additional lined piping (e.g., a knee joint) may be used to extend this distance above the ceramic ring and to place the EIJ at a selected angle with respect to vertical. An additional ceramic ring may be placed between shoulders in the EIJ. O-ring seals may be placed on the ceramic ring and in the annulus dielectrics. Pressure ports may be placed so as to indicate pressure build-up across an o-ring or other seal. A transformer may be placed so as to indicate electrical leakage current along the liner.